Small cell cancer, also known as oat cell carcinoma, is an aggressive malignancy composed of cells that are smaller than normal. It most commonly develops in the lungs but can appear in other areas of the body. This cancer is known for its rapid growth and tendency to spread quickly. The tumor cells are neuroendocrine in origin, having features of both nerve and hormone-producing cells, which influences the cancer’s behavior.
The Nature of Small Cells
At a microscopic level, the name “small cell” describes the cancer cells’ appearance. They are very small, with little cytoplasm—the substance inside a cell surrounding the nucleus. Pathologists observe these cells as having dark-staining nuclei, a high rate of cell division, and a tendency to press against each other. This appearance distinguishes it from other cancer types, such as non-small cell lung cancer, where the cells are larger.
These cells arise from the diffuse neuroendocrine system, which consists of specialized cells that have both nerve-like and hormone-producing characteristics. This dual nature explains why small cell cancers can produce and secrete hormones, such as adrenocorticotropic hormone (ACTH) or antidiuretic hormone (ADH). This hormone production can lead to a separate set of symptoms known as paraneoplastic syndromes, which are effects on the body caused by the hormones the tumor releases rather than by the physical presence of the cancer itself.
Another feature of small cell cancer is its lack of differentiation, the process by which cells mature to perform a specific function. Small cell cancers are “poorly differentiated,” meaning the cancer cells bear little resemblance to the healthy, mature cells of their original tissue. This lack of maturity contributes to the cancer’s aggressive behavior and early metastasis, or spread to distant parts of the body.
Where Small Cell Cancers Develop
The vast majority of small cell cancers originate in the lungs, where it is known as small cell lung cancer (SCLC). It begins in the central airways of the lungs and can cause symptoms as it grows and narrows these passages. Common signs associated with SCLC include a persistent cough, shortness of breath, chest pain, and unexplained weight loss. By the time symptoms appear, the cancer has often already spread beyond the lung.
While the lungs are the most common site, this cancer can also develop in other areas, a condition referred to as extrapulmonary small cell carcinoma (EPSCC). These cancers are much rarer and can arise in numerous organs. Frequent locations include the gastrointestinal tract, such as the esophagus, the genitourinary system, like the bladder and prostate, and the gynecologic system, most often the cervix.
The symptoms of extrapulmonary cancers vary depending on the organ involved. For instance, small cell carcinoma of the bladder might present with blood in the urine or changes in urinary habits. In the gastrointestinal tract, it could cause difficulty swallowing if in the esophagus or abdominal pain if in the stomach or pancreas. Small cell carcinoma of the cervix may lead to abnormal vaginal bleeding or pain.
How Small Cell Cancer is Diagnosed
The diagnostic process often begins with imaging tests after a person presents with symptoms. A chest X-ray or a computed tomography (CT) scan is a common first step, which can reveal a tumor or abnormalities in the lungs or other parts of the body.
While imaging can identify a suspicious mass, a biopsy is required for a definitive diagnosis. A biopsy involves obtaining a small tissue sample from the tumor for examination by a pathologist. For a suspected lung cancer, this sample might be collected during a bronchoscopy or a CT-guided needle biopsy if the tumor is more accessible.
The pathologist’s analysis confirms the diagnosis by looking for the distinct features of small cells, such as their small size and scant cytoplasm. Once confirmed, the cancer is staged to determine its extent. For SCLC, it is categorized into two stages: limited-stage, where cancer is confined to one side of the chest, and extensive-stage, where it has spread more widely. Staging is used to plan the most appropriate course of treatment.
Treating Small Cell Cancer
Because small cell cancer has often spread by the time it is found, treatment is guided by its stage and relies on systemic therapies. Chemotherapy is the main treatment for nearly all patients, regardless of the stage. Common regimens combine a platinum-based drug like cisplatin or carboplatin with another drug, such as etoposide.
Radiation therapy is another treatment, often used in combination with chemotherapy for limited-stage disease. This combined approach, known as chemoradiation, can be more effective for cancer confined to the chest. Radiation may also be used palliatively to relieve symptoms from tumors in the lungs, bones, or brain. Prophylactic cranial irradiation (PCI) may be offered to some patients to reduce the risk of cancer spreading to the brain.
Newer therapies have also expanded treatment options. Immunotherapy, specifically immune checkpoint inhibitors, works by helping the body’s immune system attack cancer cells. Drugs like atezolizumab and durvalumab are often combined with chemotherapy for extensive-stage SCLC. While surgery is rarely used, it might be an option in very rare cases of very early-stage SCLC.